In the field of magnetic recording media including audio tape, video tape, floppy disks, and computer tape, it is a common practice to add non-magnetic inorganic material powder such as Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, TiO.sub.2, SiO.sub.2, SiC, .alpha.-Fe.sub.2 O.sub.3, and ZnO.sub.2 for preventing the magnetic layer from damage by sliding contact with the magnetic head. Unless the type, size and amount of inorganic material particles to be added are carefully selected, such inorganic particles can damage or abrade the magnetic head or degrade the electromagnetic properties of magnetic recording media.
Often a non-magnetic inorganic material powder having high hardness is selected and added to the magnetic layer alone or in admixture for reinforcement. In accordance with the current requirement of higher performance for magnetic recording media such as video tape and digital recording tape, magnetic recording media are improved in surface smoothness by the use of microparticulate magnetic powder, the use of a highly dispersible binder or the like, which in turn, tends to deteriorate the durability, friction stability, and head seizure of the magnetic layer. It is then desired to further enhance the durability and head cleaning effect of a coating.
If conventional non-magnetic inorganic material powder commonly used in the prior art is used in high-performance magnetic recording media, there arise a number of problems including deteriorated electromagnetic properties, less reinforcement, possible damaging of the magnetic layer during durability test operation, ineffective head cleaning, increased head adhesion, and reduced outputs.
Japanese Patent Publication (JP-B) No. 59690/1992 discloses to cover non-magnetic particles at their surface with an aliphatic compound. This permits the non-magnetic particles to be uniformly dispersed in the magnetic layer for improving the abrasion resistance of the magnetic layer at no sacrifice of smoothness. This solution, however, fails to improve durability and magnetic head cleaning effect to such an extent as would meet the performance requirements of advanced magnetic recording media.
Then the inventors proposed in Japanese Patent Application Kokai (JP-A) No. 248119/1992 a magnetic recording medium having inorganic material-containing .alpha.-Al.sub.2 O.sub.3 dispersed in the magnetic layer. As compared with the conventional non-magnetic inorganic material powders, the inorganic material-containing alpha-alumina offers improved head cleaning effect to reduce debris adhesion to the head and improves the durability of the magnetic layer, resulting in a magnetic recording medium having an improved still life. A magnetic recording tape having a single magnetic layer of 3 .mu.m thick is demonstrated in Example.
This medium, however, is yet insufficient with respect to head clogging and still life. In an experiment that only tapes as manufactured (so-called virgin tape) or tapes which have been used a few times were operated with a used head for recording and replay, head clogging occurred within 12 cassettes of virgin tape because of insufficient head cleaning effect. This becomes a serious problem on practical use.